The present invention relates to a vascular endoprosthesis, such as a stent, for placement in an area of a body lumen that has been weakened by damage or disease such as by aneurysm, and in particular, to a stent adapted for placement at a neurovascular site.
Rupture of non-occlusive cerebrovascular lesions, such as intracranial saccular aneurysms or arterio-venous fistulae, are a major cause of stroke. Rupture of an aneurysm causes subarachnoid hemorrhage in which blood from a ruptured vessel spreads over the surface of the brain. About 2.5% of the United States population (4 million Americans) have an unruptured aneurysm. About 100,000 of these people suffer a subarachnoid hemorrhage. The disease is devastating, often affecting healthy people in their 40""s and 50""s, with about half of the rupture victims succumbing within a month, and with half of the survivors becoming seriously disabled as a result of the initial hemorrhage or of a delayed complication.
Neurovascular arteries are generally quite small, having diameters ranging from 2.0 to 4.0 mm in the Circle of Willis, 2.5 to 4.5 mm in the cavernous segment of the internal carotid artery, 1.5 to 3.0 mm in vessels of the distal anterior circulation, and 2.0 to 4.0 mm in the posterior circulation. The incidence of aneurysm varies with the location, with 55% occurring in the Circle of Willis, 30% in the internal carotid, 10% in the distal anterior circulation, and 5% in the posterior circulation.
Screening for these lesions and preventing rupture will lead to better clinical outcomes and lower costs. Non-invasive treatments for ruptured and unruptured lesions are preferred over surgical interventions due to lower costs, lower mortality and morbidity, and patient preference. An attractive treatment for ruptured and unruptured aneurysms is the placement of a stent within the lumen to prevent rupture or re-rupture of the lesion.
Stents formed of a helical coil or ribbon of shape-memory alloy material are known in the art. In general, such stents are formed to a desired expanded shape and size for vascular use above the transition temperature of the material. The stent is then cooled below its transition temperature and reshaped to a smaller-diameter coil suitable for catheter administration. After the stent in its contracted, smaller-diameter shape is delivered to the target site, e.g., via catheter, it is warmed by the body to above its transition temperature, causing the stent to assume its original expanded shape and size, typically a shape and size that anchors the stent against the walls of the vessels at the vascular site. Stents of this type are disclosed for example, in U.S. Pat. Nos. 4,512,338, 4,503,569, 4,553,545, 4,795,485, 4,820,298, 5,067,957, 5,551,954, 5,562,641, and 5,824,053. Also known in the art are graft-type stents designed for treating aneurysms, typically at relatively large-vessel sites, e.g., with vessel lumen sizes between about 15 and 30 mm. U.S. Pat. No. 4,512,338 is exemplary.
Stents such as disclosed heretofore have one or more of the following limitations, for purposes of the present invention:
(i) they are not capable of being advanced to a target site, such as a neurovascular site, that is accessible only along a tortuous path by a small-diameter catheter;
(ii) they may cause vessel injury due to rapid expansion at the target site;
(iii) they are not suitable for treating aneurysms in the absence of a special graft, sleeve or webbing;
(iv) they may cause thrombosis (clotting) of small vessels with low flow such as neurovascular vessels.
It would therefore be desirable to provide a stent that overcomes these limitations, and which is suitable, in one embodiment, for use in treating neuroaneurysms.
In one aspect, the invention includes a stent adapted for advancement through a catheter in a upstream to downstream direction to a target vessel site, in a contracted stent condition, and expulsion from the catheter, downstream end first, and radial expansion at the target site, to engage the walls of the vessel.
The stent is formed of a continuous helical ribbon, preferably formed of a shape-memory alloy, and has a bending-stiffness gradient along its length due to (i) a gradient of ribbon width, (ii) a gradient of ribbon thickness, and/or (iii) a gradient of size or number of openings formed in the stent ribbon. The stent has a preferred contracted-condition diameter of between about 10 and 30 mils, and a diameter in a fully expanded condition of between 40 and 125 mils.
In one general embodiment, the shape-memory alloy has a final austenite transition temperature of between about 25xc2x0 C. and 37xc2x0 C. This feature allows the stent to be moved through the catheter in a martensitic, superelastic state, and to assume its preformed, austenitic shape when expelled from the catheter. In another embodiment, the shape-memory alloy has a transition temperature Md, below which the alloy retains stress-induced martensitic properties, of greater than 37xc2x0 C. This allows the stent to be moved through the catheter in a stress-induced martensitic (SIM) state, and recover its preformed, austenite shape when released from the constraints of the catheter, at a temperature that may be substantially above the final austenite temperature. In this embodiment, the final austenite temperature may be quite low, e.g., 4xc2x0 C., or it may be room temperature or higher.
The bending-stiffness gradient may be continuous along the length of the stent, or discontinuous, e.g., having two or more separate regions, each with substantially uniform stiffness. The stiffness gradient is typically greater stiffness upstream and lesser stiffness downstream, as the stent is oriented in the catheter for delivery in an upstream-to-downstream direction.
Where the stiffness gradient is due to a gradient of ribbon width, greater ribbon width at the upstream end of the stent, and lesser ribbon width at the downstream end of the stent, the greater ribbon width is preferably (i) at least ten times the ribbon thickness and (ii) at least two times the lesser width. The greater ribbon width is effective to reduce the rate of expansion of the stent from its contracted to its radially extended condition, relative to that of a stent having uniform winding widths equal to the lesser ribbon widths, and to increase the angle of catheter bend through which the catheter can be advanced, in an upstream to downstream direction, relative to that of a stent having uniform winding widths equal to the greater ribbon width. Preferably the greater ribbon width is between 25 and 75 mils, and the lesser ribbon width, between 5 and 15 mils.
Where the stent stiffness gradient is due to fewer or smaller openings formed along the length of the helical ribbon, greater opening area in a downstream direction, the openings are preferably shaped and oriented to achieve greater stent flexibility while preserving areal coverage of the targetregion. In one general embodiment, the openings are I-beam shaped openings whose xe2x80x9cIxe2x80x9d axis is aligned transversely to the longitudinal axis of the stent in the contracted state. In another, they are Z-shaped openings whose central axis is aligned transversely to the longitudinal axis of the stent in the contracted state. The helical ribbon is effective to cover between 50% and 80% of the surface area of the vessel region containing the stent.
In a more specific embodiment, the invention includes a stent adapted for advancement through a catheter in a upstream to downstream direction to a target vessel site in a contracted stent condition, and with expulsion from the catheter, downstream end first, and radial expansion at the target site, to engage the walls of the vessel. The stent includes a continuous helical ribbon formed of a shape-memory metal having a ribbon thickness of 0.5 and 4 mils, and being effective to cover between 50% and 80% of the surface area of the vessel region containing the stent. The stent has a bending-stiffness gradient along its length due to (i) a gradient of ribbon width, (ii) a gradient of ribbon thickness; and/or a gradient of size or number of openings formed in the stent ribbon. The stent is characterized by a contracted-condition diameter of between about 10 and 30 mils, and a diameter in a fully expanded condition of between 40 and 125 mils.
In another aspect, the invention includes a catheter delivery device having a catheter for accessing an intralumenal target site, a stent of the type described above, contained within the catheter in a martensitic, superelastic state, and a catheter pusher wire for advancing the stent through the catheter in a downstream direction.
In still another aspect, the invention includes a method of treating a lesion at a neurovascular target vessel site. The method includes guiding a neuro-interventional catheter to the target site, advancing through the catheter, a stent of the type described above, and expelling the stent from the catheter at the target site, causing the stent to expand radially against the vessel walls at the target site.
The step of guiding the stent to the target site may include engaging a pusher wire releasably with the downstream end of the stent, pushing the stent through the catheter with the pusher wire, and expelling the stent from the catheter at the target site, with stent radial expansion at the target site being effective to release the stent from the pusher wire. The pusher wire can include a distal end ball adapted to be captured by the stent, with such in its contracted condition. Alternatively, the pusher wire can include a distal notch adapted to be captured by the stent, with such in its contracted condition.